This section introduces aspects that may facilitate better understanding of the present disclosure. Accordingly, the statements of this section are to be read in this light and are not to be understood as admissions about what is in the prior art or what is not in the prior art.
In a Third Generation Partnership Project (3GPP)-Long Term Evolution (LTE) network, a user equipment (UE) needs to perform measurement on its serving cell (also known as a primary cell, PCell) so as to monitor the serving cell performance. This is called as radio link monitoring (RLM) or RLM related measurement in LTE, where the downlink radio link quality of the serving cell is monitored based on a cell-specific reference signal for the purpose of indicating out-of-synchronization (OOS) status and/or in-synchronization (IS) status to higher layers.
In order to indicate OOS and IS statuses, according to the 3GPP standard, i.e. 3GPP TS 36.133 V12.5.0, a UE shall estimate the downlink radio link quality and compare it to thresholds Qout or Qin. The threshold Qout is defined as the level at which the downlink radio link cannot be reliably received and shall correspond to 10% block error rate (BLER) of a hypothetical Physical Downlink Control CHannel (PDCCH) transmission taking into account the Physical Control Format Indicator CHannel (PCFICH) errors with transmission parameters specified in Table 7.6.1-1 of 3GPP TS 36.133 V12.5.0. When the estimated BLER over an evaluation period is bigger than Qout, an OOS indication is generated. The threshold Qin is defined as the level at which the downlink radio link quality can be significantly more reliably received than at Qout and shall correspond to 2% BLER of a hypothetical PDCCH transmission taking into account the PCFICH errors with transmission parameters specified in Table 7.6.1-2 of 3GPP TS 36.133 V12.5.0. When the estimated BLER over an evaluation period is smaller than Qin, an IS indication is generated. Table 7.6.1-1 and Table 7.6.1-2 will not be represented herein for the sake of brevity. The UE may declare radio link failure (RLF) when consecutive IS indications less than a certain number are detected within a predetermined time period. This time period is started after detection of a certain number of consecutive OOS indications.
According to the 3GPP standard, e.g. 3GPP TS 36.213 V11.7.0, in non-Discontinuous Reception (DRX) operation, the downlink link quality for OOS and IS is estimated over evaluation periods as specified in 3GPP TS 36.133 V12.5.0, for example 200 ms and 100 ms, respectively. In DRX mode operation, the downlink link quality for OOS and IS is estimated over the same evaluation periods as specified in 3GPP TS 36.133 V12.5.0, which may be adjusted with a DRX cycle. The estimated radio link quality may be filtered not only on the physical layer, but also on higher layers in the evaluation period based on some configured parameters. This increases the reliability of radio link failure (RLF) detection and thus avoids unnecessary radio link failure and consequent Radio Resource Control (RRC) re-establishment.
An advanced technology with multiple antennas has been proposed for the existing wireless communications systems, which can improve coverage with beamforming and significantly increase overall system capacity with spatial multiplexing. The beamforming, where multiple antenna elements are used to form narrow beams, is efficient means for improving both data rates and capacity; while the spatial multiplexing, where propagation properties are exploited to provide multiple data streams simultaneously to one or more terminals, is another important technique for the multi-antenna technology.
With the further development of wireless communications technology, a communications network with massive beams serving a UE may appear. In such a network, the existing solution for monitoring radio link quality may not be applicable anymore due to more beams being involved.